1. Field of the Invention
The present invention relates generally to ferroelectric materials, and, more particularly, to a method of reducing the dielectric constant of such materials while preserving much of their inherent tunability.
2. Description of Related Art
Four of the most important characteristics of a ferroelectric ceramic that are desired for practical microwave phase shift devices or electronically scanned array (ESA) antennas are (1) low (.di-elect cons..sub.r .ltoreq.100) dielectric constant, (2) low (.ltoreq.0.010) loss tangent tan .delta., (3) substantial (.gtoreq.10%) tunability, and (4) stability of material properties over the operating temperature range. The material selected for a given application will, in general, be a trade-off, as not all of the properties wanted can be realized simultaneously. For example, by operating high-density barium-strontium-titanate (BST) close to its Curie temperature, a dielectric constant that exceeds 5,000 with 80 percent tunability is achievable; however, both parameters decline rapidly as the operating temperature is varied just a few degrees in either direction.
The three most important reasons for seeking materials with dielectric constants less than 100 are:
(1) Circuit dimensions and tolerances scale inversely as the square-root of dielectric constant. PA1 (2) RF losses per unit length are directly proportional to both the dielectric loss tangent and the square-root of the dielectric constant. Typically, when the dielectric constant of a material such as BST is lowered, its loss tangent is also reduced. PA1 (3) Ferroelectric ceramics with a low dielectric constant generally have material properties that exhibit better temperature stability.
This adversely impacts producibility of ferroelectric microwave devices by conventional machining techniques, especially with .di-elect cons..sub.r &gt;100.
Prior art approaches for lowering the dielectric constant employ three-dimensional thinning techniques, such as by inducing porosity in the ferroelectric material or by mixing the ferroelectric material with inert, low dielectric-constant fillers. However, as porosity or percent volume of filler increases, the polycrystalline structure of the ferroelectric ceramic becomes more and more "disconnected". By "disconnected" is meant that the ferroelectric structure is no longer continuous, with the result that the applied dc electric field moves more into the pores or filler, which effectively reduces the tunability of the composite. The applied dc electric field can be raised to compensate for this effect; however, dielectric breakdown (i.e., arcing) eventually occurs within the material before full tunability of the material can be exploited. This occurs because most of the applied dc electric field becomes impressed across the material with the lower .di-elect cons..sub.r : i.e., across the air gaps or filler rather than the ferroelectric material.
Thus, there remains a need for providing a method of reducing the dielectric constant of ferroelectric materials while retaining much of their inherent tunability.